In a well-known manner, an electrochromic element exhibits different light transmitting properties between its oxidized state and its reduced state as a result of an electrochemical reaction. The electrochromic element is used for controlling the volume of light transmission of the incident light thereon by taking advantage of the difference in the light passing properties thereof. The electrochromic element comprises a thin film formed of an electrochromic material, such as WO.sub.3 or WOD.sub.3, placed in contact with an electrolyte, such as a film-shaped lithium electrode. If the electrical voltage is forced in a direction in which the electrolyte side becomes a positive side, the state of the element is changed.
In the case of a colorless WO.sub.3 thin film, for example, if the negative voltage is applied thereto, M.sub.x +WO.sub.3ex- are formed by injection of electrons and positive ions in the electrolyte, with the thin film being changed from the colorless state into the state presenting a blue color. Consequently, if plural electrochromic elements presenting different colors in different wavelength regions are placed one on the other for taking advantage of the electric and light passing properties thereof, it becomes possible to control the volume of light transmission for the entire range of the visible light.
There has hitherto been proposed a diaphragm device for an image pickup lens system, such as a camera, which takes advantage of the above-mentioned electrical and light transmitting properties of the electrochromic elements, in e.g. JP Patent Kokai (Laid-Open) Publication No.62-198835, entitled "Diaphragm Device". The diaphragm device according to the prior-art Publication has an electrochromic element 2 sandwiched and enclosed between a pair of transparent substrates 4A, 4B, along with an electrolyte, by means of a spacer 3, and a first set of transparent electrodes 6 and a second transparent electrode 6, mounted within the interior of the transparent substrates 4A and 4B in contact with the front and reverse surfaces of the electrochromic element 2, as shown in FIGS. 7 and 8.
At least the first set of transparent electrodes 5 are made up of plural concentrically arranged transparent electrodes 5a to 5d, while the second transparent electrode plays the role of a counter-electrode for the set of transparent electrodes 5. The electrical voltage is applied across the transparent electrodes 5 and 6 via terminals 7A and 7B from the outermost transparent electrode towards the innermost transparent electrode in this order. Beginning from the annular areas of the diaphragm device 1 in register with the outer most transparent electrode 5d, the annular areas of the diaphragm device 1 in register with the transparent electrodes 5 are changed from the transparent state to the light interrupting state, in a direction of proceeding towards the annular area in register with the inner most transparent electrode 5a, for reducing the transmitting diameter for the incident light for controlling the volume of the transmitted light to a desired level.
Meanwhile, it is demanded of the diaphragm device 1 for the camera having the electrochromic element 2 to exhibit optical characteristics in which a sufficient amount of the incident light is transmitted in the transparent state and the incident light is positively interrupted in the light interrupting state. The light interrupting properties of the diaphragm device is particularly crucial if it is desired to control the depth of field by changing the size of the aperture of the diaphragm or if it is desired to decrease the aberration by reducing the effective lens diameter for improving lens characteristics.
It is now assumed that, in the prior-art diaphragm device 1, the F number during the maximum opening of the diaphragm, with the areas in register with the transparent electrodes 5a to 5d being transparent, is 1.4, the F number during the minimum opening of the diaphragm, with only the area in register with the inner most transparent electrode 5a being transparent, is 8, the ratio of light transmission of the electrochromic element 2 being 100% during the time of transmission and 1% during light interruption, respectively, and the ratio of light transmission of the transparent electrodes 5 and 6, each having the ratio of light transmission equal to 10%, is equal to (1-0.1).sup.2 =0.9.sup.2. The ratio of the light volume leaking from the light interrupting area to the light volume transmitted through the transparent area at the time of the minimum opening of the diaphragm device, that is the S/N ratio, may be calculated from the formula:
(light volume leaking from areas in register with the transparent electrodes 5b, 5c and 5d)/(light volume transmitted through an area in register with the transparent electrode 5a) (surface measures of areas in register with the transparent electrodes 5a to 5c).times.(light transmittance)/(surface measure of an area in register with the transparent electrode 5a).times.(light transmittance)
Substituting the above values into the above formula, the S/N ratio becomes equal to [(8/1.4).sup.2 -1].times.(0.01.times.0.9.sup.2)/1.times.(1.times.0.9.sup.2)=0.32.
It will be seen from the foregoing that the light volume leaking from the annular light interrupting areas in register with the transparent electrodes 5b, 5c and 5d amounts to 30% or more because of the larger surface area so that the light interrupting properties demanded of the diaphragm device 1 for the camera cannot be met. The result is that the depth of focus becomes uncontrollable due to the light leaking from the areas in register with the transparent electrodes 5b to 5d of shallow depth, while the effect which the decreased effective lens diameter might have on the progressive decreasing of the lens aberration also becomes unsatisfactory due to the leaking light.
For combatting such inconvenience, there has also been proposed a diaphragm device 9 in which two diaphragm devices 1 are stacked for improving the light interrupting characteristics, as shown in FIG. 9. In the diaphragm device 9, the parts or components which are similar to those of the diaphragm device 1 shown in FIGS. 7 and 8 are depicted by the same numerals and the corresponding description is not made herein for clarity. With the present diaphragm device 9, since the incident light is caused to pass through two layers of the electrochromic elements 2 and four layers of the transparent electrodes 6 and 7, the S/N ratio may be found from the formula (1) by [(8/1.4).sup.2 -1].times.0.01.sup.2 .times.0.9.sup.4 /1.times.(1.sup.2 .times.0.9.sup.4)=0.0032.
It may be seen that, with the diaphragm device 9, made up of six layers, that is two layers of the electrochromic elements 2 and four layers of the transparent electrodes 5, 6, the light volume leaking from the annular light interrupting areas in register with the transparent electrodes 5b, 5c and 5d during the minimum opening of the diaphragm becomes 1/100 of that of the diaphragm device 1. The result is that the light interrupting properties may be improved significantly, while the light transmitting properties in the transparent state and the light interrupting properties in the light interrupting state demanded of the diaphragm device for the image pickup lens system for the camera or the like may also be met.
However, with the diaphragm device 9, shown in FIG. 9, in which the light interrupting properties are improved by the above-described arrangement, the overall thickness is substantially twice that of the diaphragm device 1 shown in FIGS. 7 and 8, such that, due to such increase in thickness of the diaphragm device 9, the thickness of the overall optical system becomes approximately thrice that of the optical system having the diaphragm device shown in FIGS. 7 and 8.
Besides, since the incident light must pass through the four layers of the transparent electrodes 6 and 7, the light volume of the transmitted light during the light-passing state is decreased significantly. For example, if indium- thin- oxide (ITO) having superior light transmittance is used as a material for the transparent electrode, the ratio of light transmission per each layer of the transparent electrodes is decreased by about 10% due to the light absorption by the transparent electrodes, such that the light transmittance of the diaphragm device 9 in its entirety amounts to a higher value of (1-0.1).sup.4 =0.66.